Personalizable security document and method of manufacturing the same

Abstract

A personalizable security document (10) includes a layer of laser-engravable material (4) embedded in a substrate (1) of the security document (10). The layer of laser-engravable material (4) can be irradiated with laser light of a single wavelength to form a color portrait (5) of a holder of the security document (10), due to the layer of laser-engravable material (4) including thermochromic pigments that change their color in an irreversible manner upon irradiation with laser light. A reproducible result can be obtained by carefully controlling the height of the laser-engravable material layer inside the substrate (1), in particular, during manufacturing of the personalizable security document (10).

Description

Technical Field

[0001] The present disclosure generally relates to personalizable security documents, in particular, personalizable security documents such as identification documents, driver's licenses and the like, which include a portrait of a holder of the security document.Background

[0002] Generally, in the market of physical identification documents, a variety of different security features are used to ensure the document's security and integrity. In some applications, a laser-engraved image is considered vital, as the image features can be generated inside a polycarbonate substrate rather than on the surface of the substrate making it more difficult to tamper with the document. A laser-engraved security feature in a polycarbonate substrate may include a black and white (in particular, grayscale) image, a color image, or special features like the Mirage security feature of HID.

[0003] Physical identification documents such as driver's licenses, passports and the like may include a color portrait of a holder of the security document. Such color portraits are usually printed onto a surface of the security document, using commonly known printing processes.

[0004] EP 4 210 315 A1 discloses a method of generating a palette for use in a printing process. The method is used, for example, for printing a color image using a laser marking apparatus.

[0005] WO 2021 / 152018 A1 discloses a method for producing markings of different irreversible colors from the same composition.

[0006] The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems, without being limited to a particular type of security document.Summary of the Disclosure

[0007] According to one aspect of the present disclosure, a personalizable security document has a substrate having a first side and a second side opposite to the first side in a thickness direction of the substrate, the substrate including a plurality of substrate layers laminated with each other, and a layer of laser-engravable material printed on a first substrate layer of the plurality of substrate layers and embedded inside the substrate. The layer of laser-engravable material is disposed in an image-forming region of the substrate configured to have a color portrait of a holder of the security document engraved in the layer of laser-engravable material. The layer of laser-engravable material includes at least one type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with laser light of a single wavelength, wherein the color after the change varies with one or more laser parameters of the laser light. Preferably, the layer of laser-engravable material extends in a plane perpendicular to the thickness direction with a constant height in the thickness direction of between 5 µm and 10 µm, preferably between 6 µm and 8 µm.

[0008] In another aspect of the present disclosure, a method of personalizing a security document includes the steps of providing a personalizable security document in accordance with the above aspect, receiving an input image including a color portrait of a holder of the security document, retrieving a palette indicating a relationship between the one or more laser parameters and the plurality of different colors, and laser-engraving the color portrait of the holder using the laser light of a single wavelength, preferably a pulsed laser light with a wavelength of around 500 nm, by varying the one or more laser parameters.

[0009] In a further aspect, a method of manufacturing a personalizable security document includes the steps of providing a first substrate layer, printing a layer of laser-engravable material onto the first substrate layer, preferably by screen printing, flexography, offset printing, pad printing or inkjet printing, the layer of laser-engravable material including at least one type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with laser light of a single wavelength, wherein the color after the change varies with the one or more laser parameters of the laser light, and laminating the first substrate layer with at least one additional substrate layer to form a substrate of the personalizable security document. The layer of laser-engravable material is disposed in an image-forming region of the substrate configured to have a color portrait of a holder of the security document engraved in the layer of laser-engravable material, and preferably has a constant height in a thickness direction of the substrate of between 5 µm and 10 µm, preferably between 6 µm and 8µm, after lamination.

[0010] Other features and aspects of the present disclosure will be apparent from the following description and the accompanying drawings.Brief Description of the Drawings

[0011] Fig. 1 shows a plan view of an exemplary personalizable security document in accordance with the present disclosure; Fig. 2 shows a schematic cross-sectional view of the exemplary personalizable security document in Fig. 1; Fig. 3 shows a schematic cross-sectional view of another exemplary personalizable security document in accordance with the present disclosure; Fig. 4 shows a plan view illustrating different colors obtained by varying laser parameters of a laser used to personalize a personalizable security document in accordance with the present disclosure; Fig. 5 shows a plan view of an exemplary personalized security document in accordance with the present disclosure; Fig. 6 shows an exemplary flow diagram illustrating a method of personalizing a security document in accordance with the present disclosure; and Fig. 7 shows a flow diagram illustrating an exemplary method of manufacturing a personalizable security document in accordance with the present disclosure. Detailed Description

[0012] The following is a detailed description of exemplary embodiments of the present disclosure. The exemplary embodiments described herein are intended to teach the principles of the present disclosure, enabling those of ordinary skill in the art to implement and use the present disclosure in many different environments and for many different applications. Therefore, the exemplary embodiments are not intended to be, and should not be considered as, a limiting description of the scope of protection. Rather, the scope of protection shall be defined by the appended claims.

[0013] The present disclosure is based at least in part on the realization that, with advances in modern digital printers and scanners, it has become more and more difficult to prevent counterfeiting of security documents including printed images. While different solutions have been proposed to perform color laser printing inside a substrate of a security document in order to prevent counterfeiting, said solutions involve either using multiple layers of inks, which are to be excited by multiple lasers to obtain a color portrait or the like, or using a combination of laser and inkjet processes. Both solutions have the disadvantages of either being technologically complex and expensive, or unsecure due to the inkjet layer being provided on top of the substrate of the security document. It has been realized that these disadvantages can be overcome by using a single layer including thermochromic pigments, the single layer being embedded in a substrate of the security document and configured to be activated by a single laser wavelength to produce different colors depending on the laser parameters.

[0014] In this respect, it has been realized that the thickness of the layer including the thermochromic pigments plays a vital role, because the colors that are obtained after performing the laser engraving depend upon the thickness of the layer including the thermochromic pigments. Accordingly, in order to be able to obtain reproducible results, the thickness of the layer including the thermochromic pigments has to be carefully controlled and set to correspond to a predetermined reference thickness based on which the obtainable colors were determined.

[0015] It has been realized that a constant predetermined thickness of the layer including the thermochromic pigments can be advantageously obtained by applying an ink including the thermochromic pigments, optionally mixed with a varnish or the like, on a substrate layer of the substrate forming the security document, and laminating the substrate layer with the ink including the thermochromic pigments with one or more additional substrate layers in order to form the substrate of the security document. In this respect, it has been realized that it may be necessary to take into consideration the effect that the process of laminating the different substrate layers may result in a change, in particular, a thickness reduction, of the layer including the thermochromic pigments. Accordingly, it may be necessary to apply the layer including the thermochromic pigments with a thickness that is larger than the desired thickness after lamination.

[0016] It has also been realized that the reproducibility of the colors can be further increased by processing the applied layer including the thermochromic pigments prior to laminating the different substrate layers. In this respect, it has been realized that specific curing techniques are advantageous. For example, thermal curing can be performed by heating the substrate layer including the layer with the thermochromic pigments. Alternatively or additionally, curing with ultraviolet (UV) light can be used to obtain a layer having the desired properties.

[0017] Further, it has been realized that the color quality can be increased by providing the layer including the thermochromic pigments on top of an opaque, for example, white background layer. This increases the visibility of the color portrait to be formed, and also the image quality.

[0018] It has also been realized that, although it may be possible to obtain a plurality of different colors using a single layer with thermochromic pigments, the available colors may nevertheless be limited to specific colors or a specific number of discrete colors. Here, it has been realized that additional colors, which cannot be directly obtained from a single layer with thermochromic pigments can be generated by performing halftoning. In such a manner, additional colors that are perceived by an observer can be created, for example, by performing vector error diffusion halftoning or similar processes.

[0019] It has also been realized that, in order to further improve the appearance of the color portrait to be generated, gamut mapping of color values of an input image to be engraved to a palette that is generated based on the obtainable colors of the layer with the thermochromic pigments can be used.

[0020] Finally, it has also been realized that the number of obtainable colors can be increased by providing a further layer of laser-engravable material with at least one type of thermochromic pigments below the first layer, resulting in different color combinations depending on the laser parameters. For example, the first layer can be provided on a substantially transparent substrate layer, and the further layer can be provided on an opaque, preferably white background layer such that both layers can be processed by a laser at the same time. In other examples, both layers can be provided on top of each other, i.e., contact each other. In such cases, the layers may be provided on the same substrate layer, or they may be provided on opposing surfaces of two adjacent substrate layers.

[0021] As used herein, the term "thermochromic pigments" refers to pigments that have a property of irreversibly changing color depending on the exposure to light, in particular, laser light of a specific wavelength. This photo-induced process results in an activation of the pigments such that the pigments change their state by electronic excitation. If the pigments have a plurality of different excitation states, a plurality of different colors can be obtained, wherein the different colors are not continuous, i.e., discrete colors that are not obtained as a gradual variation of the obtained color with a gradual change in laser parameters. This is different from other materials such as, for example, the Mirage security feature of HID. At the same time, the laser energy is low enough that the material in which the pigments are embedded is not modified. Pigments or materials including such pigments are known, such that a detailed description will be omitted herein. Examples are thermochromic liquid crystals (organic products that change state upon irradiation with laser light), microencapsulated thermochromic pigments (so-called leuco-dyes made of, for example, a colorant, a weak acid, and a solvent), and hybrid and / or inorganic thermochromic materials. For example, pigments developed by the company OLIKROM can be used. Here, it is immediately evident that, in the present disclosure, the pigments are configured such that the photo-induced activation is irreversible, i.e., after activation, the pigments maintain their new color, and do not revert back to their original color.

[0022] Fig. 1 shows a plan view of an exemplary personalizable security document 10 in accordance with the present disclosure. As shown in Fig. 1, security document 10 includes a substrate 1, for example, having a substantially rectangular shape. In particular, as shown in Fig. 1, security document 10 is a personalizable security document, i.e., does not yet include an image, for example, a photograph or the like, of a person to which the document belongs. Such an image can be formed later, in an image-forming region 3 provided on a first side S1 of substrate 1, as will be described in more detail below.

[0023] As shown in Fig. 2, which is a cross-sectional view of substrate 1, substrate 1 also has a second side S2 opposite to first side S1 in a thickness direction d of substrate 1. Image-forming region 3 is provided on first side S 1 of substrate 1 and is configured to have a color portrait 5 (see Fig. 5) of a holder of security document 10 engraved in the same. Here, the expression "on the first side" is to be interpreted such that a region or layer is provided on one of the opposing sides of security document 10. However, it is not necessary that the region or layer is provided on an outer surface of substrate 1 on the respective sides.

[0024] As can be seen in Figs. 1 and 2, personalizable security document 10 further has a layer of laser-engravable material 4 embedded inside substrate 1 in image-forming region 3. The layer of laser-engravable material 4 includes at least one type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with laser light of a single wavelength, where the color after the change varies with one or more laser parameters of the laser light. As already described, thermochromic pigments are pigments which exhibit an induced color change upon irradiation with light, in particular, laser light having a specific wavelength, where the resulting color (of a plurality of discrete colors) depends on the other laser parameters that are used, for example, laser power, laser speed, repetition rate, focus depth, etc.

[0025] The thermochromic pigments are provided in a matrix of support material, and the laser parameters are selected such that the properties of the support material are not changed by the laser light. As such, it is only the color of the thermochromic pigments inside the layer of laser-engravable material 4 that is changed by the laser light. Here, it is important to note that the color-change that is described herein is different from a change that is obtained when a physical structure of a material layer such as a transparent substrate layer or the like is modified by being burnt using a laser having a high power. Although a grayscale image could be formed in such a manner, for example, in a top layer of substrate 1, such a formation of a grayscale image by burning of the material of the layer is not to be understood as having a color-change in accordance with the present disclosure. As used herein, the pigments that exhibit the color-change change their color from a first, in particular, chromatic color (for example, violet) to a second, different color (for example, yellow, orange, red, grey).

[0026] As shown in Fig. 2, layer of laser engravable material 4 extends in a plane perpendicular to thickness direction d with a constant height h in thickness direction d. The constant height h is, in particular, between 5 µm and 10 µm, preferably between 6 µm and 8 µm.

[0027] In the exemplary embodiment shown in Fig. 2, substrate 1 includes a plurality of substrate layers 31, 32, 33, 34 laminated with each other. Layer of laser-engravable material 4 is printed on a first substrate layer 31 of the plurality of substrate layers 31, 32, 33, 34. Here, any appropriate printing technique can be used to apply layer of laser-engravable material 4 onto substrate 31, for example, screen printing, flexography, offset printing, pad printing or inkjet printing. What is important is that the printing, as well as the subsequent manufacturing of substrate 1, is carefully controlled to result in the desired height h in thickness direction d of layer of laser-engravable material 4. Layer of laser-engravable material 4 may be formed by thermochromic pigments arranged in a matrix of an appropriate support material that can be easily applied onto substrate layer 31, for example, in the form of an ink or any other coating.

[0028] Substrate layers 31, 32, 33, 34 may be made from any appropriate material that is commonly used for forming security documents, for example, polycarbonate, thermoplastic polyurethane (TPU) and / or polyvinyl chloride (PVC). As will be described later, substrate layers 31, 32, 33, 34 may be combined by laminating the same in order to form substrate 1 in a known manner.

[0029] Fig. 4 shows an exemplary manner of producing different colors by irradiating laser light of a single wavelength onto layer of laser-engravable material 4. As shown on the left side in Fig. 4, layer of laser-engravable material 4 (in other words, image-forming region 3) may be subdivided into a plurality of image units 6, for example, pixels, that form image-forming region 3 and can be selectively irradiated with laser light having different laser parameters. Depending on the selected laser parameter, each image unit or pixel 6 may have one of a plurality of different colors a to i, as shown on the right side in Fig. 4. In such a manner, by providing a reference layer of laser-engravable material 4 having a constant height h of, in particular, between 5 µm and 10 µm, preferably between 6 µm and 8 µm, and including a specific type of thermochromic pigments, different ranges of laser parameters may be scanned in order to generate the plurality of different colors a to i. In such a manner, a palette of obtainable colors can be generated for the given height h and the given type of thermochromic pigments. Color portrait 5 can then be formed in personalizable security document 10 with reference to the previously obtained palette, by determining a color value of each image unit or pixel 6 in color portrait 5 and controlling the laser in an appropriate manner to generate, for each image unit or pixel 6, the required color value. This will be described in more detail in the following.

[0030] In some embodiments, first substrate layer 31 is opaque, preferably, having a white color, and layer of laser-engravable material 4 is printed on a surface of substrate layer 31 on first side S1. In such a manner, the colors obtained by irradiating the laser light onto layer of laser-engravable material 4 are viewed against or on a white background, which increases the quality of the resulting color image, in particular, a contrast and a perceived color tone of the same.

[0031] In other embodiments, however, first substrate layer 31 may be transparent, and layer of laser-engravable material 4 may be printed on one of a surface of first substrate layer 31 on first side S1 and a surface of first substrate layer 31 on second side S2. Also in this case, however, the image that is formed inside layer of laser-engravable material 4 may be viewed against a white or another opaque background. However, the additional advantage of further protecting the layer of laser-engravable material 4 inside substrate 1 is obtained. To this end, substrate 1 further includes a second substrate layer 33 arranged below first substrate layer 31, with second substrate layer 33 being opaque, preferably, having a white color.

[0032] It will be appreciated that many different configurations of substrate layers 31, 32, 33, 34 can be used. For example, it will be immediately evident that it is not necessary that image-forming region 3 is provided on first side S1 of substrate 1. Instead, image-forming region 3 can also be provided on second side S2 of substrate 1. Further, it will be appreciated that the substrate layer 31, on which layer of laser-engravable material 4 is provided, is not necessarily the topmost or the second layer from the top, but can be any appropriate layer inside substrate 1, as long as layer of laser-engravable material 4 is embedded in substrate 1 after forming the same and can be engraved using the appropriate laser light.

[0033] Fig. 3 shows another exemplary security document 10, in particular, a substrate 1 of the same, in accordance with the present disclosure. In the example shown in Fig.3, security document 10 further comprises a second layer of laser-engravable material 8 embedded in substrate 1 in image-forming region 3, overlapping first layer of laser-engravable material 4. The second layer of laser-engravable material 8 is formed on a different substrate layer than the layer of laser-engravable material 4, with a height h2 in thickness direction d of between 5 µm and 10 µm, preferably between 6 µm and 8 µm. In particular, second layer of laser-engravable material 8 includes at least one second type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with the laser light of a second wavelength. The color after the change varies with the one or more laser parameters of the laser light. The at least one second type of thermochromic pigments is different from the at least one type of thermochromic pigments in the layer of laser-engravable material 4. A distance between first layer of laser-engravable material 4 and second layer of laser-engravable material 8 may be between 0 µm and 200 µm, preferably between 50 µm and 150 µm.

[0034] In such a manner, two different types of thermochromic pigments can be provided inside image-forming region 3, and can be simultaneously irradiated with the laser light of the single wavelength to produce different colors. Here, it will be immediately evident that, by varying the laser parameters other than the laser wavelength, a larger number of perceived colors can be obtained, because the pigments in each of the two layers change their color, resulting in many possible combinations of colors that are obtained, with each combination resulting in a perceived color of color portrait 5. Accordingly, the total number of colors, as well as the color gradients that can be obtained, can be increased to further improve the image quality.

[0035] Again, the substrate layer on which the second layer of laser-engravable material 8 is formed may be opaque, preferably, may have a white color, and may be arranged below first substrate layer 31 (which, in this case, is at least partially transparent). This again results in that color portrait 5 after laser engraving is viewed on or against a white background.

[0036] Although it is shown in Fig. 3 that second layer of laser-engravable-material 8 is provided on a surface of substrate layer 33 on first side S1, it will be appreciated that this is not limiting. In other examples, second layer of laser-engravable material 8 may be provided on an opposite side of substrate layer 33, with substrate layer 33 being transparent.

[0037] In other embodiments, layer of laser-engravable material 4 may be provided on a surface of substrate layer 31 on second side S2, i.e., opposing second layer of laser-engravable material 8. As such, after lamination and forming of substrate 1, layer of laser-engravable material 4 and second layer of laser-engravable material 8 may contact each other.

[0038] As shown in Figs. 2 and 3, substrate 1 may further include a transparent protective layer 32 covering first substrate layer 33. This assures that layer of laser-engravable material 4 is embedded inside substrate 1 and can be protected from outside influences even in a case where layer of laser-engravable material 4 is provided on a surface of the second layer from the top on first side S1.

[0039] In the following, an exemplary method of personalizing a security document such as security document 10 will be described with reference to Fig. 6. In a first step 110, a personalizable security document 10 in accordance with the present disclosure is provided.

[0040] In a second step 120, an input image 16 including a portrait of a holder of security document 10 is received. Here, input image 16 may be any appropriate color image including the portrait of the holder of security document 10.

[0041] In step 130, a palette 12 indicating a relationship between the one or more laser parameters and the plurality of different colors is retrieved, for example, from a memory of a printing apparatus that is used to perform the personalization. As previously described, palette 12 has been generated in advance, for example, on the basis of irradiating a reference layer of laser-engravable material 4 having a constant height h of between 5 µm and 10 µm, preferably between 6 µm and 8 µm, and measuring the color values of the obtained colors in an appropriate color space such as RGB or CIELAB. Accordingly, palette 12 may indicate the relationship between the one or more laser parameters for irradiating the layer of laser-engravable material 4 and the resulting different colors.

[0042] In step 140, color portrait 5 of the holder of security document 10 is laser engraved in image-forming region 3 using the laser light of a single wavelength. Here, it is preferred that a pulsed laser light is used. It will be appreciated that the wavelength has to be set such that it is capable of activating the thermochromic pigments in layer of laser-engravable material 4. In some embodiments, a wavelength of around 500 run can be used to perform the laser engraving. The parameters that can be varied to obtain different colors may include one or more of the following: laser speed, laser power, repetition rate, focus depth, etc. An exemplary laser speed is between 100 mm / s and 4000 mm / s, and an exemplary repetition rate is between 50 kHz and 100 kHz.

[0043] It will be appreciated that there may be cases where input image 16 includes colors that are not obtainable by varying the laser parameters from laser-engravable material 4. In such cases, the method may include a further step 135 of performing gamut mapping to map color values of input image 16 to the plurality of colors obtainable from palette 12. The process of gamut mapping is well-known to the skilled person such that it will not be described in detail herein. For example, US 2007 / 0097389 A1 describes such a gamut mapping. In simple terms, the gamut mapping results in that the color values of input image 16 are mapped to the closest color values (i.e., with a shortest color distance in the given color space) of the colors obtainable from palette 12.

[0044] As initially mentioned, the method may further include an optional step 136 of performing halftoning, i.e., laser engraving color portrait 5 by halftoning using palette 12 and, optionally, the gamut-mapped color values of input image 16. The process of halftoning is also well-known, such that it will not be described in detail. For example, US 5,070,413 A discloses a technique that is known as "vector error diffusion", which could be employed. Essentially, halftoning results in that colors other than the exemplary directly obtainable colors a to i shown in Fig. 4 can be generated in image-forming region 3, by appropriately combining the directly obtainable colors a to i in a specific manner.

[0045] If the process of halftoning is used, it will be appreciated that, in the gamut-mapping, also the colors that are not directly obtainable, but can be obtained by halftoning, are used.

[0046] In order to reduce the number of colors in palette 12, while at the same time obtaining the best possible image quality, the techniques described in EP 4 210 315 A1 can be used in order to maximize the color gamut for a given number of colors in palette 12.

[0047] An exemplary method of manufacturing personalizable security document 10 will be described in the following with reference to Fig. 7. In step 210, a first substrate layer 31 may be provided. Here, substrate layer 31 may be configured in the above-described manner, for example, as a polycarbonate layer, a PVC layer or a PU layer.

[0048] In a second step 220, a layer of laser-engravable material 4 is printed onto first substrate layer 31, preferably by screen printing, flexography, offset printing, pad printing or inkjet printing. Layer of laser-engravable material 4 includes at least one type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with laser light of a single wavelength. The color after the change varies with the one or more laser parameters of the laser light.

[0049] In step 230, first substrate layer 31 is laminated with at least one additional substrate layer 32, 33, 34 to form substrate 1 of personalizable security document 1.

[0050] After lamination, layer of laser-engravable material 4 is disposed in an image-forming region 3 of substrate 1 configured to have a color portrait 5 of a holder of security document 10 engraved in the same. Layer of laser-engravable material 4 has a constant height h in a thickness direction of substrate 1 of between 5 µm and 10 µm, preferably between 6 µm and 8 µm. In other words, the method includes a step of disposing layer of laser-engravable material 4 in image-forming region 3 of substrate 1, and controlling or adjusting a height of layer of laser-engravable material 4 in thickness direction d of substrate 1 to a defined height, for example, between 5 µm and 10 µm, preferably between 6 µm and 8 µm.

[0051] · In some embodiments, the method includes a step 225 of curing layer of laser-engravable material 4, which may be a mixture of a thermochromic ink and a varnish, prior to laminating the same, preferably, by heating first substrate 31 with layer of laser-engravable material 4 to a temperature between 90°C and 120°C, preferably 110°C, for one to three minutes, preferably two minutes, optionally in combination with irradiating infrared light, and / or performing ultraviolet curing for one to ten minutes, for example, in a subsequent step.

[0052] Further, in some embodiments, the method includes a further step 226 of providing a second substrate layer 33, and a step 227 of printing a second layer of laser-engravable material 8 onto second substrate layer 33, preferably by screen printing, flexography, offset printing, pad printing or inkjet printing, wherein second layer of laser-engravable material 8 includes at least one second type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with the laser light of a single wavelength, with the color after the change varying with the one or more laser parameters of the laser light. The at least one second type of thermochromic pigments is different from the at least one type of thermochromic pigments in the layer of laser-engravable material 4.

[0053] In a step 228, first substrate layer 31 is laminated with second substrate layer 33. After lamination, second layer of laser-engravable material 8 is disposed in image-forming region 3 of substrate 1 and has a constant height h2 in thickness direction d of substrate 1 of between 5 µm and 10 µm, preferably between 6 µm and 8 µm.

[0054] It will be appreciated that the above-described curing can also be advantageously applied to second layer of laser-engravable material 8 prior to lamination.

[0055] Further, as initially mentioned, the process of laminating the different substrate layers may result in a change, in particular, a decrease in the heights h and h2 of the laser-engravable material layers 4, 8. As such, the above methods may include a step of applying the respective laser-engravable material layers with heights h' and h" that are greater than the heights h and h2 after lamination, i.e., adjusting the heights of the applied layers of laser-engravable material to be greater than the heights after lamination.

[0056] As already mentioned, at least one additional substrate layer 32, 33, 34 may be provided, which includes a third substrate layer 33 that is opaque, preferably, has a white color, with the method further comprising a step 239 of arranging third substrate layer 33 below first substrate layer 31 prior to laminating the same.Industrial applicability

[0057] With the above-described techniques, a personalizable security document 10 that can be personalized by forming a color portrait 5 of a holder of the security document in an image-forming region 3 of substrate 1 using a single laser in a convenient manner is achieved. Here, image-forming region 3 may be a region for forming a primary portrait of the holder, or it may be part of a security feature in which a secondary portrait, i.e., a copy of the primary portrait is formed. As satisfactory results can only be obtained by carefully controlling, in particular, the height of the layer of laser-engravable material, and said layer of laser-engravable material 4 is embedded inside substrate 1, it becomes difficult for unauthorized parties to counterfeit security document 10.

[0058] It will be appreciated that the foregoing description provides examples of the disclosed systems and methods. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the general disclosure.

[0059] Recitation of ranges of values herein are merely intended to serve as a shorthand method for referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All method steps described herein can be performed in any suitable order, unless otherwise indicated or clearly contradicted by the context.

[0060] Although the preferred embodiments of the present disclosure have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.

Claims

1. A personalizable security document (10) comprising: - a substrate (1) having a first side (S1) and a second side (S2) opposite to the first side in a thickness direction (d) of the substrate (1), the substrate (1) including a plurality of substrate layers (31, 32, 33, 34) laminated with each other; and - a layer of laser-engravable material (4) printed on a first substrate layer (31) of the plurality of substrate layers (31, 32, 33, 34) and embedded inside the substrate (3), the layer of laser-engravable material (4) being disposed in an image-forming region (3) of the substrate (1) configured to have a color portrait (5) of a holder of the security document (10) engraved in the layer of laser-engravable material (4), wherein the layer of laser-engravable material (4) includes at least one type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with laser light of a single wavelength, the color after the change varying with one or more laser parameters of the laser light.

2. The security document of claim 1, wherein the layer of laser-engravable material (4) extends in a plane perpendicular to the thickness direction (d) with a constant height (h) in the thickness direction of between 5 µm and 10 µm, preferably between 6 µm and 8 µm.

3. The security document of claim 1 or 2, wherein the first substrate layer (31) is opaque, preferably, has a white color, and the layer of laser-engravable material (4) is printed on a surface of the first substrate layer (31) on the first side (S1).

4. The security document of claim 1 or 2, wherein the first substrate layer (31) is transparent, and the layer of laser-engravable material (4) is printed on one of a surface of the first substrate layer (31) on the first side (S1) and a surface of the first substrate layer (31) on the second side (S2).

5. The security document of any one of claims 1 to 4, wherein the substrate (1) further includes a second substrate layer (33) arranged below the first substrate layer (31), the second substrate layer (33) being opaque, preferably, having a white color.

6. The security document of any one of claims 1 to 5, wherein the security document (10) further comprises a second layer of laser-engravable material (8) embedded inside the substrate (1) in the image-forming region (3), the second layer of laser-engravable material (8) being formed on a different substrate layer than the layer of laser-engravable material (4) with a height (h2) in the thickness direction of between 5 µm and 10 µm, preferably between 6 µm and 8 µm, wherein the second layer of laser-engravable material (8) includes at least one second type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with the laser light of a single wavelength, the color after the change varying with the one or more laser parameters of the laser light, the at least one second type of thermochromic pigments being different from the at least one type of thermochromic pigments in the layer of laser-engravable material (4).

7. The security document of claim 6, wherein the substrate layer on which the second layer of laser-engravable material (8) is formed is opaque, preferably, has a white color, and is arranged below the first substrate layer (31).

8. The security document of claim 6 or 7, wherein the layer of laser-engravable material (4) and the second layer of laser-engravable material (8) contact each other.

9. The security document of any one of claims 1 to 8, further comprising a transparent protective layer (32) covering the first substrate layer (33).

10. A method of personalizing a security document (10), comprising: - providing a personalizable security document (10) in accordance with any one of claims 1 to 9; - receiving an input image (16) including a color portrait (5) of a holder of the security document (10); - retrieving a palette (12) indicating a relationship between the one or more laser parameters and the plurality of different colors; and - laser engraving the color portrait (5) of the holder of the security document (10) in the image-forming region (3) using the laser light of a single wavelength, preferably a pulsed laser light with a wavelength of around 500 nm, by varying the one or more laser parameters.

11. The method of claim 10, further comprising at least one of: - performing gamut mapping to map color values of the input image (16) to a plurality of colors obtainable from the palette (12); and - laser engraving the color portrait (5) by halftoning using the palette (12) and, optionally, the gamut-mapped color values of the input image (16).

12. A method of manufacturing a personalizable security document (10), the method comprising: - providing a first substrate layer (31); - printing a layer of laser-engravable material (4) onto the first substrate layer (31), preferably by screen printing, flexography, offset printing, pad printing or inkjet printing, the layer of laser-engravable material (4) including at least one type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with laser light of a single wavelength, the color after the change varying with one or more laser parameters of the laser light; and - laminating the first substrate layer (31) with at least one additional substrate layer (32, 33, 34) to form a substrate (1) of the personalizable security document (1), wherein the layer of laser-engravable material (4) is disposed in an image-forming region (3) of the substrate (1) configured to have a color portrait (5) of a holder of the security document (10) engraved in the layer of laser-engravable material (4), preferably, wherein the layer of laser-engravable material (4) has a constant height (h) in a thickness direction of the substrate (1) of between 5 µm and 10 µm, preferably between 6 µm and 8 µm, after lamination.

13. The method of claim 12, further comprising: - curing the layer of laser-engravable material (4) prior to laminating the same, preferably, by heating the first substrate layer (31) with the layer of laser-engravable material (4) to a temperature between 90 °C and 120 °C, preferably 110 °C, for 1 to 3 minutes, preferably 2 minutes, and / or performing ultraviolet curing for 1 to 10 minutes.

14. The method of claim 12 or 13, further comprising: - providing a second substrate layer (33); - printing a second layer of laser-engravable material (8) onto the second substrate layer (33), preferably by screen printing, flexography, offset printing, pad printing or inkjet printing, wherein the second layer of laser-engravable material (8) includes at least one second type of thermochromic pigments configured to irreversibly change color from an initial color to a plurality of different colors upon irradiation with the laser light of a single wavelength, the color after the change varying with the one or more laser parameters of the laser light, the at least one second type of thermochromic pigments being different from the at least one type of thermochromic pigments in the layer of laser-engravable material (4); and - laminating the first substrate layer (31) with the second substrate layer (33), wherein, after lamination, the second layer of laser-engravable material (8) is disposed in the image-forming region (3) of the substrate (1) and has a constant height (h2) in the thickness direction of the substrate (1) of between 5 µm and 10 µm, preferably between 6 µm and 8 µm.

15. The method of any one of claims 12 to 14, wherein the at least one additional substrate layer (32, 33, 34) includes a third substrate layer (33), the third substrate layer (3) being opaque, preferably, having a white color, the method further comprising arranging the third substrate layer (33) below the first substrate layer (31) prior to laminating the same.

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